Machines used to break up concrete and asphalt pavement and other hard surfaces have a plurality of tools mounted on a cutting wheel which is forced against the surface to be broken. Each tool has an elongate steel body at the forward end of which is a tungsten carbide cutting tip. When the wheel rotates, the tools are carried through a circular orbit and the tungsten carbide tips penetrate the hard surface with each tip removing a small amount of material, thereby advancing the cut.
The tools suffer wear as a result of being moved against the hard material being cut and have to be replaced at regular intervals. Each time the tools are replaced, the machine is taken out of service for a lengthy period of time. Machines used to break up concrete and asphalt roadways are kept in continuous operation through the work day except for when the tools are being replaced, and it is not uncommon to replace the tools in such machines two or three times during a work day. The frequency with which tools have to be replaced and the time consumed during the replacement process therefore reduce the efficiency of the machine and increase the cost of its operation.
The cost of replacement tools and the efficiency with which the tools cut the hard abrasive material also effect the economic efficiency of the machine. To minimize tool costs it desirable that the components of the tool, namely the tool body and the tungsten carbide cutting tip, have comparable endurance to wear. The energy needed to operate the machines, and therefor the cost of operation, increases if the cutting tips become too blunt before the tool body has become sufficiently worn to require replacement. Both the cost of the tool and the cutting efficiency of the tool are important factors in maximizing the efficiency of the tools.
Tool failure can occur as a result of the failure the braze material holding the tungsten carbide tip into the seat at the forward end of the tool body. To prevent failure of the braze and the dislodging of the insert, the hardened inserts of such tools should have a base diameter of at least 0.700 inches.
Theoretically, the life of the cutting insert will be increased by providing a larger sized insert, however enlarging all diameters of a currently available one piece tungsten carbide cutting tip will reduce the efficiency of the machine because the tip will rapidly become blunt. Since the tungsten carbide is the most expensive element in the tool, increasing the size of the insert will also increase tool cost. On the other hand, the metal which makes up the tool bodies is subject to wash away causing the tool to fail as aggregate of the hard material cut by the machine erodes away the metal of the tool body behind the tungsten carbide tip.
It has long been recognized that the useful life of a tool can be substantially extended by increasing the hardness of the tungsten carbide from which the cutting inserts are made. Efforts to make a tool having a harder insert, however, have not been successful partly because harder grades of tungsten carbide are more brittle and tend to fracture, and partly because the harder grades of tungsten carbide are more difficult to manufacture. A harder insert is manufactured by using particles of tungsten carbide and cobalt and having smaller grain sizes of tungsten carbide with a higher concentration of tungsten carbide and a corresponding lower concentration of cobalt. It is the cobalt which cements sintered tungsten carbide together and to compensate for the reduction of cobalt in the product the particles must be more uniformly compacted together prior to sintering to reduce the intergranular porosity. If the particles are not uniformly compacted the completed insert will have less dense areas, or porosity, and be subject to failure.
One effort to provide an insert which is made of a harder grade which is less subject to fracture is depicted in FIGS. 15-17 of U.S. Pat. No. 5,551,760 to Sollami. The insert depicted in Sollami has a cylindrical core and surrounding the core is an annular collar made of a softer grade of tungsten carbide. The core and the collar are bonded together with braze material. One difficulty with the Sollami insert has been the difficulty of properly assembling the parts of the insert and maintaining the part in their correct relationship during the brazing process. Specifically, the weight of the tungsten carbide collar is insufficient to push the collar through the liquefied braze material until it seats properly at the bottom of the seat. The proper seating of the collar therefore requires at least one more step in the manufacturing process. It would be desirable, therefore, to provide an improved two piece insert consisting of a hard elongate core and a softer annular collar in which the parts would fit together as a unit so as become properly seated during the brazing operation.